Petalosarsia declivis Sars 1865

Petalosarsia declivis (Sars, 1865)

(Figs. 1, 2)

Petalopus declivis Sars, 1865: 197.

Petalomera declivis Sars, 1883, 13.

Petalosarsia declivis Stebbing, 1893: 308; Sars, 1900, 77–79, pl. 54; Given, 1965, 222, Fig. 5; Bacescu & Muradian, 1974, 224–227, Figs. 5–7.

Material examined. Two ovigerous females, 3.84, 3.72 mm, dissected (NSMT-Cr 22061), off Abashiri, Okhotzk Sea, 44°03.58ˏN, 144°34.83ˏE – 44°03.60ˏN, 144°37.36ˏE, 104 m (KT-01-14, St. AB-7), 20 September, 2001; 4 ovigerous females, 3.13–3.58 mm, 2 specimens dissected (NSMT-Cr 22062), off Setana, 42°25.37ˏN, 139°45.91ˏ– 42°25.01ˏN, 139° 46.10E, 122–130 m (KT-04-20, St. ST2-2), 18 September, 2004; 1 ovigerous female, 3.20 mm, dissected (NSMT-Cr 22063), off Rishiri, northern part of the Sea of Japan, 45°12.5ˏN, 140°48.9ˏE, 472 m, 4 August 2009 (collected during cruise by R/V Soyo-maru of Fisheries Research Agency, Japan).

Description. Ovigerous females (Figs. 1, 2). Body heavily calcified. Carapace with numerous reticular depressions, length 1.0–1.3 times width, length 1.6–1.8 times depth, length 0.31–0.35 times total body length (Fig. 1 A). Dorsal surface flat and broadest at hind end; dorsolateral carina with a tooth on anterior end, meeting posteriorly with a transverse ridge near hind margin; very weak lateral ridge present on each side, running parallel to dorsolateral carina; between dorsolateral carina and lateral ridge below forming very shallow sulcus; inferior margin smooth, with no tooth; frontal margin of pseudorostral lobes, between 2 teeth at anterior end of dorsolateral carina, round, 0.35–0.45 times greatest width of carapace. Pseudorostrum short, 0.09–0.13 times length of carapace. Width of rounded eye lobe 0.137–0.157 times greatest width of carapace. Anterolateral angle and antennal notch obsolete. Pereon 0.60–0.70 times carapace. Pleon short, 0.44–0.47 times total body length (Figs. 1 A, B).

Antenna 1 (Fig. 1 C), 1st article of peduncle 1.05–1.4 times combined length of 2nd and 3rd articles; main flagellum 0.48–0.53 times basal article of peduncle; 1st article 1.2–1.5 times combined length of remaining 2 articles; accessory flagellum minute. Antenna 2 (Fig. 1 D) bi-articulate, with plumose terminal seta on distal article, with 2 plumose setae on basal article. Labium with a blunt process on tip (Fig. 1 E). Left and right mandibles with 8 and 7 setae respectively (Fig. 1 F). Maxilla 1 (Fig. 1 G) outer lobe with 10 stout setae on distal margin; inner lobe with 2 simple and 1 tridentate setae on distal margin. Maxilla 2 shown in Fig. 1 H. Maxilliped 1 with 1 branchial lobule, basis shorter than remaining distal articles (Fig. 1 I). Maxilliped 2 (Fig. 1 J), basis 1.0–1.2 times combined length of following 4 articles, with semicircular plate on ventral surface and with a group of 3–5 spines on inner dorsal surface. Maxilliped 3 (Fig. 1 K), basis 1.0–1.3 times distal articles together, with 3–4 plumose setae on inner margin; carpus with 3–4 plumose setae and hyaline lamella on inner margin.

Pereopod 1 (Fig. 1 L), basis 1.2–1.4 times combined length of ischium and carpus, with 11–13 plumose setae on inner margin; carpus flat, 2.0–2.2 times width and 2.3–2.7 times as long as dactylus; dactylus subequal in length to propodus. Pereopod 2 (Fig. 1 M), basis 0.9–1.0 times distal articles together, with 4–5 plumose setae on inner margin; dactylus subequal to carpus, less than twice as long as propodus, with 5 naked setae on distal half of article. Pereopod 3 (Fig. 2 A, B), basis 1.5–1.8 times distal articles together, with 2–4 plumose setae; single article minute exopod, with plumose and naked setae on tip. Pereopod 4 (Fig. 2 C, D), basis 1.0–1.1 times distal articles together, with 2 plumose setae; locus for exopod on outer margin of basis slightly swollen or decalcified, with a couple of plumose and naked setae. Pereopod 5 (Fig. 2 E), basis 0.6–0.7 times distal articles together.

Uropod (Fig. Fib. 2F) peduncle 1.07–1.20 times pleonite 6, 0.68–0.79 times exopod length, 0.58–0.63 times endopod length, with a short seta on inner distal margin; exopod 0.80–0.91 times endopod length, with 3 setae on tip; endopod with numerous hairs on inner margin, armed with 0 or 1 spiniform setae on inner margin and 2 terminal setae. Telson semicircular, 0.41–0.55 times pleonite 6.

Distribution. North Atlantic boreal and Arctic region, NW Canada, Detroit de Davis, Iceland, Scandinavia, Spitsbergen, Beloe More, Novaya Zemlya, Kamchatka, the Sea of Japan. Okhotzk Sea, Alaska, 18– 472 m.

Remarks. Female specimens examined in the present study agree well with the description of Petalosarsia declivis (Sars, 1900), except that the frontal margin of the carapace is not acute. In addition, the basal article of antenna 2 in the Japanese specimens is furnished with two plumose setae. In the Atlantic specimen described by Sars (1900) and other Petalosarsia species from Japan and the Sulu Sea, this article bears a plumose seta.

Sars (1900) showed a tiny bi-articulate exopod on pereopod 3 of a female from the Northeast Atlantic. However, Bacescu and Muradian (1974) observed that exopods on pereopods 3 and 4 were absent in the specimens from the Northwest Atlantic. Japanese specimens were characterized by a minute, single article exopod on pereopod 3 and total absence of an exopod on pereopod 4 except for a couple of setae that may be remnant terminal setae of the exopod. It is possible that the exopods on pereopods 3 and 4 may show variation between individuals, although this was not observed.

Petalosarsia declivis from Alaska, relatively close to northern Japan, shows two lateral ridges under the dorsolateral carina of the carapace, according to Given (1965). Japanese female specimens have only 1 weak lateral ridge like the specimens from the Northwest Atlantic, described by Bacescu and Muradian (1974). The morphological variation mentioned above suggests that the name P. declivis, as currently used, may represent a species complex. Detailed study of populations from the various localities is necessary to determine if the variation is due to divergence or phenotypic plasticity.